1. Field of the Invention
The present disclosure generally relates to photodetectors and more particularly to single photon detectors.
2. Description of the Related Art
A photodetector is a device that provides an electrical voltage or electrical current output signal when light is incident thereon. There are two basic types of photodetectors: linear detectors and quantum detectors. Linear detectors provide an output signal that is a linear function of the incident light intensity of average optical power. Quantum detectors provide an output signal upon detection of photons of the incident light.
A single-photon detector is a qunatum detector that can detect one incident photon at a time. Commercially available single photon detectors detect photons in the visible and shorter wavelength optical regions of the electromagnetic spectrum. These commercially available detectors include silicon avalanche photodiodes (Si APDs), such as part number C30954 from EGandG Optoelectronics. A typical Si APD has a responsivity of 70 A/W (amps/watt) for photons with wavelengths of 900 nm, which drops to 36 A/W for photons with wavelengths of 1064 nm. Currently available Si APDS are not sensitive enough to detect photons with wavelengths longer than 1100 nm.
Characteristics of hot-electron photodetectors that are fabricated from superconducting NbN (niobium nitride) films are discussed in K. S. Il""in, I. I. Milostnaya, A. A. Verevkin, G. N. Gol""tsman, E. M. Gershenzon, and Roman Sobolewski, xe2x80x9cUltimate Quantum Efficiency of A Superconducting Hot-Electron Photodetector,xe2x80x9d Applied Physics Letters Vol. 73, No. 26 (Dec. 18, 1999), pages 3938-3940 and in K. S. Il""in, M. Lindgren, M. Currie, A. D. Semenov, G. N. Gol""tsman, Roman Sobolewski, S. I. Chereduichenko, and E. M. Gershenzon, xe2x80x9cPicosecond Hot-Electron Energy Relaxation in NbN Superconducting Photodetectors,xe2x80x9d Applied Physics Letters Vol. 76, No. 19 (May 8, 2000), pages 2752-2754. Both publications are incorporated herein by reference. Some of the authors of the above mentioned articles are also inventors of this disclosure. While the first article suggests that xe2x80x9cNbN HEPs should be able to detect single quanta of the far-infrared radiation and successfully compete as single-photon detectors with SIS-tunnel devicesxe2x80x9d (Applied Physics Letters, Vol. 73, No. 26 at p. 3940), there is no further relevant disclosure. The second article discusses the intrinsic response times of the hot-electron effect in NbN""s, which applies to both linear and quantum NbN photodetectors.
The present disclosure addresses the above mentioned limitation of prior art photodetectors by providing a single-photon, time-resolving detector with good quantum efficiency for photons in the wavelengths from the visible to the far infrared spectral region.
In one embodiment, the single-photon detector includes a strip of superconducting material. The superconductor is biased with electrical current that is near the superconductor""s critical current. The superconductor provides a discernible output pulse signal upon absorption of a single incident photon. In one embodiment, the superconductor is a narrow strip of NbN film. In another embodiment, the superconductor has a meandering shape to increase its surface area and thus also the probability of absorbing a photon from a light source.
The present single-photon detector can be used in a variety of applications including free-space and satellite communications, quantum communications, quantum cryptography, weak luminescence, and semiconductor device testing.